The long term objectives of the proposed research are: (1) to elucidate the mechanism of regulation of O-antigen synthesis in Salmonella; and (2) to determine the physiological significance of such regulation, especially as it may relate to pathogenicity and host-parasite interactions. Specifically, genetic and molecular biological approaches will be used to determine the mechanism of positive regulation of the rfbP gene of S. typhimurium by the product of the rfaH gene and to identify possible mechanisms of regulation of rfaH. The rfbP gene encodes the enzyme for the first committed step in O-antigen biosynthesis. Molecular genetic approaches will be used to verify that rfaH acts by antitermination of transcription and to identify cis-acting sequences of rfbP that are required. Mutants that suppress the rfaH-negative phenotype will be isolated and characterized genetically and physiologically to identify whether accessory proteins that may be required for rfaH action. Finally, the hypothesis that rfaH is itself genetically regulated will be tested by isolation of mutants in the postulated regulatory pathway; it is postulated that such regulation may contribute to survival and virulence of Salmonella in the animal host.